We associate all these things with a holiday abroad and a long, sizzling day at the beach. But really, many of them — okay, maybe not the parasol — should make an appearance on even cloudy days in London. It may be a tad excessive to carry around a bag stuffed with supplies for sunburn and heatstroke while strolling through Marylebone in Spring, but you would be doing your health, and particularly your eyes, a favour, by keeping on hand things like water, lip balm, and, crucially, a great pair of sunnies.

But that begs the question, what qualifies as a high-quality pair of shades? Is it enough that they’re designer and look cool with their dark tint? Or do they need to meet certain requirements and be of a particular shape, size, colour, and even style?

1. Ultraviolet can be ultra dangerous

If your sunglasses do one thing, make sure they block 99-100 percent of UV rays. Ultraviolet rays sit on the high end of the spectrum of electromagnetic radiation, just below X-rays, and have been linked to everything from skin cancer and cataracts to macular degeneration and photokeratitis. Ensuring your shades block the vast majority of ultraviolet radiation means you protect your eyes from UVA, UVB, and UVC rays, all known for their uniquely damaging effects on the eyes and skin.

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2. Grandma always knows best

A hat and sunglasses might sound like advice your grandma would give you, but it couldn’t be more accurate. That’s because the skin around the eyes is the thinnest and most fragile on the face, not to mention it’s under the strain of 22 muscles and 10,000 blinks a day.

Yep, grandma knows best. A whopping 90 percent of premature ageing around the eyes is caused by UV damage, so if you want to still look good when you’re her age, forget what’s trendy and cover up with a wide-brimmed hat and wrap around or oversized shades.

3. A polarising position on polarised lenses

While UV blocking shades protect the eyes and the sensitive tissue around them, polarised lenses reduce glare coming off asphalt, water, snow, and other surfaces so you can comfortably see what’s in front of you. While they’re not a must (no matter what the salesperson says), they may help in reducing eye strain and stop you squinting in the sun all the time.

4. Darker is not always better

Even though they appear so, darker lenses aren’t necessarily better for your eyes — or your street cred — than others. In fact, they can actually be worse as by making the world around you darker, they cause your pupils to dilate and let in more radiation.

Lenses with coloured tints like green, amber, or grey can be useful for sportspeople, for example, as they increase contrast. But again, they’re not better at protecting your eyes unless they have the adequate UV protection.

5. Shades are not just seasonal

Up there with myths like you can’t get sunburnt through windows and breakfast is the most important meal of the day, is the idea that if it’s cool or cloudy outside, you don’t need to cover up from the sun. But the reality is up to 80 percent of the sun’s UV rays can pass through clouds, causing many people to suffer serious burns and UV damage even on overcast days.

Keep your eyes behind shades and on the UV index forecast on the Met Office website to ensure you’re adequately covered up on days of strong UV radiation. And if you’re planning a ski holiday, remember that snow can reflect UV rays and the higher the altitude, the greater the exposure.

The main takeaway is that shades are not just a timeworn fashion statement; they’re a sign you’re serious about your long-term eye health. Still, that’s no excuse to wear them at indoor parties, cafes, or long after the sun has gone down. Sorry, but not cool.